FIG. 4.

Promoter activity in J774 cells as measured by chloramphenicol resistance. Monolayers of J774 cells grown on glass coverslips were infected with DP-L2986, DP-L2989, or DP-L2812. Chloramphenicol (10 μg/ml) (cm) was added at 2.5 h postinfection. Monolayers were lysed at the indicated time points, and the number of bacteria per coverslip was determined. Results are expressed as the mean number of bacteria ± the standard deviation for three coverslips per time point. One of three experiments with similar results is shown. The MIC of chloramphenicol for L. monocytogenes 10403S grown in J774 cells was determined to be 5 μg/ml by measuring the number of CFU per monolayer in the presence and absence of increasing doses of chloramphenicol at various time points after infection (data not shown).